feat(linter): implement noUnusedExpressions
#7511
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🦋 Changeset detectedLatest commit: 3407340 The changes in this PR will be included in the next version bump. This PR includes changesets to release 13 packages
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WalkthroughAdds a new nursery lint rule Suggested reviewers
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🧹 Nitpick comments (2)
.changeset/unused-underlings-underperform.md (1)
5-5: Use past tense in changeset description.Per the coding guidelines, changeset descriptions should use past tense for what you did.
-Added nursery rule [`noUnusedExpressions`](https://biomejs.dev/linter/rules/no-unused-expressions/) to flag expressions used as a statement that is neither an assignment nor a function call. +Added nursery rule [`noUnusedExpressions`](https://biomejs.dev/linter/rules/no-unused-expressions/) that flags expressions used as a statement that is neither an assignment nor a function call.crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs (1)
206-229: Consider extracting directive-checking logic for reuse.The directive detection logic here could potentially be useful for other rules. Consider moving it to a shared utility module if other rules need similar functionality.
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⛔ Files ignored due to path filters (8)
crates/biome_configuration/src/analyzer/linter/rules.rsis excluded by!**/rules.rsand included by**crates/biome_diagnostics_categories/src/categories.rsis excluded by!**/categories.rsand included by**crates/biome_js_analyze/src/lint/nursery.rsis excluded by!**/nursery.rsand included by**crates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/invalid.tsx.snapis excluded by!**/*.snapand included by**crates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/valid.tsx.snapis excluded by!**/*.snapand included by**crates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/validDirectives.js.snapis excluded by!**/*.snapand included by**packages/@biomejs/backend-jsonrpc/src/workspace.tsis excluded by!**/backend-jsonrpc/src/workspace.tsand included by**packages/@biomejs/biome/configuration_schema.jsonis excluded by!**/configuration_schema.jsonand included by**
📒 Files selected for processing (7)
.changeset/unused-underlings-underperform.md(1 hunks)crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs(1 hunks)crates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/invalid.tsx(1 hunks)crates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/valid.tsx(1 hunks)crates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/validDirectives.js(1 hunks)crates/biome_rule_options/src/lib.rs(1 hunks)crates/biome_rule_options/src/no_unused_expressions.rs(1 hunks)
🧰 Additional context used
📓 Path-based instructions (5)
crates/biome_*/**
📄 CodeRabbit inference engine (CLAUDE.md)
Place core crates under /crates/biome_*/
Files:
crates/biome_rule_options/src/lib.rscrates/biome_rule_options/src/no_unused_expressions.rscrates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/validDirectives.jscrates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rscrates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/invalid.tsxcrates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/valid.tsx
**/*.rs
📄 CodeRabbit inference engine (CONTRIBUTING.md)
Format all Rust source files before committing (just f)
Files:
crates/biome_rule_options/src/lib.rscrates/biome_rule_options/src/no_unused_expressions.rscrates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
.changeset/*.md
📄 CodeRabbit inference engine (CONTRIBUTING.md)
.changeset/*.md: In changeset files, only use #### or ##### headers; avoid other header levels
Changeset descriptions should use past tense for what you did (e.g., "Added...")
Describe current Biome behavior in present tense within changesets (e.g., "Biome now supports...")
For bug fixes in changesets, start with a link to the issue (e.g., "Fixed #1234: ...")
When referencing rules or assists in changesets, include links to their documentation pages
Include a minimal code block in the changeset when applicable to demonstrate the change
End every sentence in the changeset description with a period
Files:
.changeset/unused-underlings-underperform.md
crates/biome_*_{syntax,parser,formatter,analyze,factory,semantic}/**
📄 CodeRabbit inference engine (CLAUDE.md)
Maintain the per-language crate structure: biome_{lang}_{syntax,parser,formatter,analyze,factory,semantic}
Files:
crates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/validDirectives.jscrates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rscrates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/invalid.tsxcrates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/valid.tsx
**/tests/**
📄 CodeRabbit inference engine (CLAUDE.md)
Place test files under a tests/ directory in each crate
Files:
crates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/validDirectives.jscrates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/invalid.tsxcrates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/valid.tsx
🧠 Learnings (8)
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/crates/biome_rule_options/lib/**/*.rs : Define per-rule options types in `biome_rule_options` crate (one file per rule, e.g., `lib/use_my_rule.rs`)
Applied to files:
crates/biome_rule_options/src/lib.rscrates/biome_rule_options/src/no_unused_expressions.rs
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/crates/biome_rule_options/lib/**/*.rs : Options types must implement serialization/deserialization and schema support using derives: `Serialize`, `Deserialize`, `Deserializable`, and `#[cfg_attr(feature = "schema", derive(schemars::JsonSchema))]`
Applied to files:
crates/biome_rule_options/src/no_unused_expressions.rs
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/**/src/{lint,assist}/**/*.rs : In declare_lint_rule! macros, set `version: "next"` for new or updated rules
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/crates/biome_js_analyze/lib/src/{lint,assist}/**/*.rs : When banning globals (e.g., `noConsoleLog`), check the semantic model to avoid false positives from locally shadowed bindings
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rscrates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/invalid.tsx
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/**/src/{lint,assist}/**/*.rs : When deprecating a rule, add `deprecated: "<reason>"` to `declare_lint_rule!`
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/**/src/{lint,assist}/**/*.rs : Follow rule naming conventions: use `no<Concept>` to forbid and `use<Concept>` to mandate; prefer consistent prefixes (e.g., `noDuplicate<Concept>`, `useConsistent<Concept>`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/**/src/{lint,assist}/**/*.rs : Code blocks in rule docs must specify language; invalid snippets require `expect_diagnostic`; use `options`/`full_options`/`use_options` markers as appropriate
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/crates/**/tests/specs/**/*.jsonc : Snapshot test `.jsonc` files must contain an array of code snippets (strings); these run in script mode (no ESM syntax)
Applied to files:
crates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/invalid.tsxcrates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/valid.tsx
🧬 Code graph analysis (1)
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs (2)
crates/biome_analyze/src/rule.rs (1)
same(241-246)crates/biome_rowan/src/ast/mod.rs (1)
cast_ref(142-151)
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🔇 Additional comments (7)
crates/biome_rule_options/src/lib.rs (1)
203-203: LGTM!The module export is correctly placed in alphabetical order.
crates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/validDirectives.js (1)
5-5: LGTM!Good test coverage for directive prologues. The test correctly verifies that string literals at the beginning of a script are treated as directives and don't trigger the rule.
crates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/valid.tsx (1)
1-26: LGTM!Comprehensive test coverage for valid cases including assignments, function calls, delete operations, void expressions, directives, and JSX elements.
crates/biome_js_analyze/tests/specs/nursery/noUnusedExpressions/invalid.tsx (1)
1-62: LGTM!Excellent test coverage for invalid cases, properly covering all the edge cases mentioned in the rule documentation.
crates/biome_rule_options/src/no_unused_expressions.rs (1)
1-6: LGTM!The options struct follows all the required patterns with proper derives and serde attributes. Good foundation for future rule options.
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs (2)
14-167: LGTM!Well-documented rule with comprehensive examples covering both invalid and valid cases, plus clear explanation of directive handling.
252-301: Solid pattern matching for expression types.The exhaustive match on expression types with clear categorisation of allowed vs disallowed expressions is well-structured. The recursive handling of TypeScript wrapper expressions is particularly neat.
CodSpeed Performance ReportMerging #7511 will not alter performanceComparing Summary
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Actionable comments posted: 2
🧹 Nitpick comments (1)
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs (1)
204-206: Tiny docs nit: spelling.“preceeded” → “preceded”.
Apply this diff:
-/// preceeded by any non-directive nodes. +/// preceded by any non-directive nodes.
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⛔ Files ignored due to path filters (4)
crates/biome_cli/src/execute/migrate/eslint_any_rule_to_biome.rsis excluded by!**/migrate/eslint_any_rule_to_biome.rsand included by**crates/biome_configuration/src/analyzer/linter/rules.rsis excluded by!**/rules.rsand included by**packages/@biomejs/backend-jsonrpc/src/workspace.tsis excluded by!**/backend-jsonrpc/src/workspace.tsand included by**packages/@biomejs/biome/configuration_schema.jsonis excluded by!**/configuration_schema.jsonand included by**
📒 Files selected for processing (1)
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs(1 hunks)
🧰 Additional context used
📓 Path-based instructions (3)
crates/biome_*_{syntax,parser,formatter,analyze,factory,semantic}/**
📄 CodeRabbit inference engine (CLAUDE.md)
Maintain the per-language crate structure: biome_{lang}_{syntax,parser,formatter,analyze,factory,semantic}
Files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
crates/biome_*/**
📄 CodeRabbit inference engine (CLAUDE.md)
Place core crates under /crates/biome_*/
Files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
**/*.rs
📄 CodeRabbit inference engine (CONTRIBUTING.md)
Format all Rust source files before committing (just f)
Files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
🧠 Learnings (7)
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/**/src/{lint,assist}/**/*.rs : In declare_lint_rule! macros, set `version: "next"` for new or updated rules
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/crates/biome_js_analyze/lib/src/{lint,assist}/**/*.rs : When banning globals (e.g., `noConsoleLog`), check the semantic model to avoid false positives from locally shadowed bindings
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/**/src/{lint,assist}/**/*.rs : Code blocks in rule docs must specify language; invalid snippets require `expect_diagnostic`; use `options`/`full_options`/`use_options` markers as appropriate
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/**/src/{lint,assist}/**/*.rs : Follow rule naming conventions: use `no<Concept>` to forbid and `use<Concept>` to mandate; prefer consistent prefixes (e.g., `noDuplicate<Concept>`, `useConsistent<Concept>`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/**/src/{lint,assist}/**/*.rs : When deprecating a rule, add `deprecated: "<reason>"` to `declare_lint_rule!`
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/**/src/{lint,assist}/**/*.rs : Set the `language` field in `declare_lint_rule!` to the primary language (`js`, `jsx`, `ts`, or `tsx`) the rule targets
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
📚 Learning: 2025-09-10T08:05:22.867Z
Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-09-10T08:05:22.867Z
Learning: Applies to crates/biome_analyze/**/src/{lint,assist}/**/*.rs : Recommended rules with domains are enabled only when users enable the matching domains; use `domains` metadata judiciously
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs
🧬 Code graph analysis (1)
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs (2)
crates/biome_analyze/src/rule.rs (3)
sources(569-572)same(241-246)recommended(554-557)crates/biome_rowan/src/ast/mod.rs (1)
cast_ref(142-151)
⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (25)
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- GitHub Check: Test Node.js API
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🔇 Additional comments (1)
crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs (1)
160-166: Rule metadata looks spot on.
version: "next",sources: …same(), andrecommended: falseall align with the project conventions.
| /// Determines if a node exists at the top-level based on its `parent` node. | ||
| fn is_at_top_level(parent: &JsSyntaxNode) -> bool { | ||
| AnyJsRoot::can_cast(parent.kind()) | ||
| || TsModuleBlock::can_cast(parent.kind()) | ||
| || JsBlockStatement::cast_ref(parent).is_some_and(|block| { | ||
| block | ||
| .syntax() | ||
| .parent() | ||
| .is_some_and(|parent: JsSyntaxNode| AnyJsFunction::can_cast(parent.kind())) | ||
| }) | ||
| } |
There was a problem hiding this comment.
Don’t treat TS namespace/module blocks as directive contexts.
Directive prologues exist only for Script/Module roots and Function bodies. Allowing them inside TsModuleBlock deviates from JS semantics and can hide unused string expressions in namespace/module bodies.
Apply this diff:
fn is_at_top_level(parent: &JsSyntaxNode) -> bool {
AnyJsRoot::can_cast(parent.kind())
- || TsModuleBlock::can_cast(parent.kind())
|| JsBlockStatement::cast_ref(parent).is_some_and(|block| {
block
.syntax()
.parent()
.is_some_and(|parent: JsSyntaxNode| AnyJsFunction::can_cast(parent.kind()))
})
}📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| /// Determines if a node exists at the top-level based on its `parent` node. | |
| fn is_at_top_level(parent: &JsSyntaxNode) -> bool { | |
| AnyJsRoot::can_cast(parent.kind()) | |
| || TsModuleBlock::can_cast(parent.kind()) | |
| || JsBlockStatement::cast_ref(parent).is_some_and(|block| { | |
| block | |
| .syntax() | |
| .parent() | |
| .is_some_and(|parent: JsSyntaxNode| AnyJsFunction::can_cast(parent.kind())) | |
| }) | |
| } | |
| /// Determines if a node exists at the top-level based on its `parent` node. | |
| fn is_at_top_level(parent: &JsSyntaxNode) -> bool { | |
| AnyJsRoot::can_cast(parent.kind()) | |
| || JsBlockStatement::cast_ref(parent).is_some_and(|block| { | |
| block | |
| .syntax() | |
| .parent() | |
| .is_some_and(|parent: JsSyntaxNode| AnyJsFunction::can_cast(parent.kind())) | |
| }) | |
| } |
🤖 Prompt for AI Agents
In crates/biome_js_analyze/src/lint/nursery/no_unused_expressions.rs around
lines 231-241, the current is_at_top_level function incorrectly treats
TsModuleBlock as a directive-context; remove the
TsModuleBlock::can_cast(parent.kind()) check so only AnyJsRoot and function-body
block checks count as top-level/directive contexts, ensuring namespace/module
bodies are not treated as directive prologues.
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Summary
Added nursery rule
noUnusedExpressionsto flag expressions used as a statement that is neither an assignment nor a function call. See #7364The rule is a port of the
no-unused-expressionsrule from ESLint. No options have been implemented yet. The only difference is that the ESLint rule hasenforceForJSXby default set tofalse, while I opted to set it totrueby default. I couldn't think of any reason not to enable it.Invalid examples
Valid examples
Test Plan
Tests added.
Docs
Rule documentation is included in the rule itself.